The Institute for Electrical and Electronics Engineers (IEEE), in resolution IEEE 802.11, also referred as 802.11, has defined a plurality of specifications which are related to wireless networking. With current existing 802.11 standards, such as 802.11(a), (b), (g), which can support up to 54 Mbps data rates, either in 2.4 GHz or in 5 GHz frequency bands, the IEEE standards body created a new task group, 802.11n, to support higher than 100 Mbps data rates. Among them being discussed are specifications for closed loop feedback mechanisms by which a receiving station may feed back information to a transmitting station to assist the transmitting station in adapting signals, which are sent to the receiving station. In closed loop feedback systems, a transmitting station may utilize feedback information from a receiving station to transmit subsequent signals in what is called beamforming. Beamforming is a technique to steer signals to a certain direction for the receiver to receive it more reliably with less noise and interference. Compounded with demands for new features and capabilities, various proposals for new 802.11n based feedback mechanisms are emerging to address the demand for these new features and capabilities. For example, there exists a demand for the introduction of new capabilities, which may enable a receiving mobile terminal to feedback pertinent information to a transmitting mobile terminal. This feedback of pertinent information may enable the transmitting mobile terminal to adapt its mode of transmission based upon the feedback information provided by the receiving mobile terminal. As with any communication system, a major goal is to enable the transmitting mobile station to achieve a higher information transfer rate to the receiving mobile terminal, while simultaneously achieving a lower packet error rate (PER). Notwithstanding, there are no existing methodologies that adequately address these shortcomings and the demand for these new features and capabilities in WLANs.
MIMO systems employing beamforming may enable the simultaneous transmission of multiple signals occupying a shared frequency band, similar to what may be achieved in code division multiple access (CDMA) systems. For example, the multiplicative scaling of signals prior to transmission, and a similar multiplicative scaling of signals after reception, may enable a specific antenna at a receiving mobile terminal to receive a signal which had been transmitted by a specific antenna at the transmitting mobile terminal to the exclusion of signals which had been transmitted from other antenna. However, MIMO systems may not require the frequency spreading techniques used in CDMA transmission systems. Thus, MIMO systems may make more efficient utilization of frequency spectrum.
One of the challenges in beamforming is that the multiplicative scale factors which are applied to transmitted and received signals may be dependent upon the characteristics of the communications medium between the transmitting mobile terminal and the receiving mobile terminal. A communications medium, such as a radio frequency (RF) channel between a transmitting mobile terminal and a receiving mobile terminal, may be represented by a transfer system function, H. The relationship between a time varying transmitted signal, x(t), a time varying received signal, y(t), and the systems function may be represented as shown in the following equation:y(t)=H×x(t)+n(t)where n(t) represents noise which may be introduced as the signal travels through the communications medium and the receiver itself. In MIMO systems, the elements in the equation above may be represented as vectors and matrices. If a transmitting mobile terminal comprises M transmitting antenna, and a receiving mobile terminal comprises N receiving antenna, then y(t) may be represented by a vector of dimensions N×1, x(t) may be represented by a vector of dimensions M×1, n(t) by a vector of dimensions N×1, and H may be represented by a matrix of dimensions N×M. In the case of fast fading, the transfer function, H, may itself become time varying and may thus also become a function of time, H(t). Therefore, individual coefficients, hij(t), in the transfer function H(t) may become time varying in nature.
In MIMO systems which communicate according to specifications in IEEE resolution 802.11, the receiving mobile terminal may compute H(t) each time a frame of information is received from a transmitting mobile terminal based upon the contents of a preamble field in each frame. The computations which are performed at the receiving mobile terminal may constitute an estimate of the “true” values of H(t) and may be known as “channel estimates”. For a frequency selective channel there may be a set of H(t) coefficients for each tone that is transmitted via the RF channel. To the extent that H(t), which may be referred to as the “channel estimate matrix”, changes with time and to the extent that the transmitting mobile terminal fails to adapt to those changes, information loss between the transmitting mobile terminal and the receiving mobile terminal may result.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.